The term “microwave” is used herein to mean electromagnetic radiation at a frequency lying in the range 1 gigahertz (GHz) to 100 GHz, approximately.
Self-sustained electron-avalanche discharging (known as “multipactor”, “multipaction” or “multipacting”) constitutes an undesirable phenomenon that can occur in microwave waveguide devices operating in a vacuum under high power conditions (typically above 1 kilowatt (kW)). Such discharging is caused by free electrons that, on being accelerated by the electric field oscillating at microwave frequency, strike the walls of the waveguide and therefore cause secondary electrons to be emitted. When the oscillation frequency of the electrons resonates with the frequency of the electric field, the number of electrons grows exponentially, thereby inducing harmful effects such as losses and a high level of noise, and possibly even damage to the waveguide. A more thorough discussion of this phenomenon can be found in the article by M. Ludovico, G. Zarba, L. Accatino, and D. Raboso “Multipaction analysis and power handling evaluation in waveguide components for satellite antenna applications”, Exp., Vol. 1, No. 2, December 2001.
The microwave waveguide filters used in satellites, in particular in the outlet sections of multichannel transmitters, but also in the inlet sections of receivers, in diplexers, in orthomode junctions, in antenna power feed systems, etc., are strongly affected by self-sustained electron-avalanche discharges. It is therefore highly desirable to prevent such discharges in the space and telecommunications industry, particularly since there is a trend towards increasing the power levels of the signals that are to be transmitted through a given waveguide device.
Several solutions have been proposed to this problem, but none of them gives full satisfaction.
A first solution, known from the article “High frequency breakdown characteristics of various electrode geometries in air” by W. G. Dunbar, D. L. Schweickart, J. C. Hotwath, and L. C. Walk, Conference Record of the 1998 Twenty-Third International Power Modulator Symposium, 1998, Jun. 22-25, 1998, pp. 221-224, consists merely in using waveguides presenting a relatively large minimum spacing between the E planes: that ensures that the maximum electric field in the waveguide is kept below a discharge threshold value. Unfortunately, that solution degrades the filtering properties of devices; in addition it leads to increasing their weight and their size, both of which are very troublesome in the context of space applications.
Another solution consists in maintaining within the waveguide a gas at a pressure that is sufficiently high, so as to reduce the mean free path length of electrons, thereby increasing the threshold power at which self-sustained electron-avalanche discharges appear. That solution also presents drawbacks, since the presence of the gas can lead to corona discharges and constitutes a potential source of passive intermodulation (PIM). In addition, pressurization equipment increases the weight, the size, and the cost of the system significantly.
To shorten the mean free path length of electrons, it is also possible to fill the waveguide with a solid dielectric or a dielectric in the form of a foam, but that increases the level of losses. In this context, reference can be made to the article by R. A. Kishek and Y. Y. Lau “Multipactor discharge on a dielectric”, Proceedings of the 1997 Particle Accelerator Conference”, Vol. 3, May 12-16, 1997, pp. 3198-3200, Vol. 3.
R. L. Geng and H. Padamsee (PAC[13], 1999, p. 429) have proposed using electric and/or magnetic fields that are constant in order to disturb the paths followed by electrons and prevent them from entering into resonance with the microwave frequency field. Unfortunately, that solution requires special equipment to generate the constant fields, thereby increasing the weight, the size, and the cost of the system.
The same authors have also proposed opening slots in the walls of the waveguide (“Multipacting in a rectangular waveguide”, R. Geng, H. Padamsee, V. Shernelin, Proceedings of the Particle Accelerator Conference, Chicago 2001). A drawback of that solution is the risk of losing radiation through said slots.
Another solution known in the prior art, e.g. proposed by Y. Saito (“Surface breakdown phenomenon in aluminum RF windows”, IEEE Transactions on Dielectrics and Electrical Insulation, Vol. 2, No. 2, April 1995) and by K. Primdahl et al. (“Reduction of multipactor in RF ceramic windows using a simple titanium-vapor deposition system”, K. Primdahl, R. Kustom, J. Maj, Proceedings of the 1995 Particle Accelerator Conference, 1995) consists in using suitable coatings and/or surface treatments, which are nevertheless liable to introduce high loss levels.
Consequently, there exists a need to increase the power that can be injected into a microwave filter without running the risk of inducing a self-sustained electron-avalanche discharge, while avoiding degrading its electrical properties, such as loss levels in the passband, bandwidth, cut-off band attenuation, and/or noise and intermodulation levels, or at least while ensuring that these degradations are kept to an acceptable level, and to do so without excessively increasing the cost, the weight, and/or the size of the filter.
The invention provides a solution to at least one of the above-mentioned problems.
The principle on which the invention is based is using a waveguide presenting two opposite walls that are not mutually parallel, while presenting a cross-section that is constant at least locally, i.e. constant over a certain length, thereby enabling the paths followed by secondary electrons to be modified in such a manner as to increase greatly the threshold at which self-sustained electron-avalanche discharges appear. This effect was observed for the first time by E. Chojnacki (Physical review special topics—accelerators and beams, Vol. 3, 032001-2000) for waveguides of constant section operating at radiofrequency (RF) (500 megahertz (MHz)) under steady or quasi-steady conditions.
In general, it is expected that a modification to the geometry of a waveguide will greatly disturb the electrical properties of a device constructed using said waveguide, and in particular its frequency response: on this topic reference can be made to the above-discussed effect of increasing the minimum spacing. That does not give rise to particular problems in the application considered by Chojnacki, i.e. transmission under steady or quasi-steady conditions, substantially at a single frequency, but it can be completely unacceptable for a filter.
Nevertheless, the inventors have discovered that by replacing waveguide segments of rectangular section in a conventional microwave filter with waveguide segments of cross-sections that present two opposite sides that are not mutually parallel, and by appropriately modifying certain dimensions of the various elements of said filter, it is possible to obtain a transfer function that is substantially identical to that of the initial filter, at least within a working band. It is thus possible to increase the ability of the filter to withstand self-sustained electron-avalanche discharges while nevertheless conserving its filter properties. In addition, the solution of the invention makes it possible to keep the size and the weight of the filter substantially constant. Even if the cost of fabrication it is likely to be slightly greater than for a conventional filter, the extra cost remains less than that associated with most solutions known in the prior art.
The inventors have also developed a design method for determining the dimensional modifications that need to be made to an initial conventional filter in order to maintain its filter properties in spite of rectangular waveguide segments being replaced by waveguide segments having walls that are not parallel.